Tungsten-halogen lamps have been employed in the European automotive industry for a number of years and have begun to penetrate the automotive market in the United States in recent years. Such lamps demonstrate at least four practical advantages, when compared with sealed beam lamps conventionally utilized in the automotive industry: (1) the light emitted is whiter; (2) the lamp can be much smaller in dimensions than the conventional lamp while producing an equivalent or even greater amount of light; (3) the intensity of the illumination remains virtually constant throughout the life of the lamp; and (4) the lamp exhibits a significantly longer working life than the conventional incandescent lamp.
Nevertheless, because tungsten-halogen lamps operate at much higher temperatures than the standard incandescent lamp, glasses useful as envelopes for such lamps must be thermally stable and resist dimensional deformation at those temperatures. Envelopes have been prepared from fused quartz and 96% silica glass compositions inasmuch as those materials possess strain points and thermal stabilities for in excess of the lamp operating temperatures. Disadvantageously, however, those glasses are quite difficult to form and lampwork and, because of their very low coefficients of thermal expansion, require special sealing techniques to introduce the lead wires into the lamps.
Consequently, extensive research has been conducted to formulate glass compositions demonstrating melting and forming characteristics rendering them suitable for use in the mass production of lamp envelopes, while concurrently displaying the chemical and physical properties demanded for that application. Much of that research has centered upon glass compositions in the alkaline earth aluminosilicate system. Because tungsten-halogen lamp envelopes are most economically fabricated from sections of glass tubing, it would be most desirable for the glasses proposed for that application to display the thermal stability and viscosity parameters operable in the Vello high speed tube drawing process. Hence, such glasses will exhibit a viscosity at the liquidus of at least 50,000 poises and, preferably, in excess of 60,000 poises and a liquidus temperature no higher than 1130.degree. C. The glasses must resist devitrification for relatively long periods of time when the glass is subjected to a temperature at or somewhat below its liquidus (thermal stability), the strain point of the glass will be at least 665.degree. C., and the coefficient of thermal expansion (0.degree.-300.degree. C.) will be about 42-45.times.10.sup.-7 /.degree.C.